This invention relates to a polyimide composition, preferably in the form of a film or sheet, containing a polyorganosiloxane for improving atomic oxygen resistance (AOR).
Data from space shuttle flights has revealed that many surface protective materials commonly used on satellite systems such as polyesters, polyimides, and polyurethanes, undergo drastic degradation by the atomic oxygen environment of the low earth orbit (LEO). The atmosphere interacts with the space craft surface materials, causing changes in their surface and bulk properties. The degradation manifests itself as material mass loss and changes in surface morphology, strength and thermal/optical properties. For example, polyimide film when exposed to atomic oxygen on shuttle flights turned from glossy amber to a flat yellow, and measurably lost thickness.
A preprint of the 32nd International SAMPE Symposium, April 6-9, 1987, discloses on page 590 imidized polyimide-polydimethylsiloxane segmented copolymers derived from 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 4,4'-diaminodiphenyl ether and from 30 to 50% of polydimethylsiloxane oligomer segments. The polysiloxane-modified polyimide copolymers show improved weatherability to aggressive oxygen environments and are used as protective coatings for aerospace applications. In contrast, the polyimide compositions of the invention do not contain polysiloxane copolymer segments.
A preprint of the 18th International SAMPE Technical Conference, October 7-9, 1986, discloses on page 1018 (Table 1) a 25% polysiloxane/45% polyimide composition which exhibits low reaction efficiency when exposed to atomic oxygen in LEO. The composition of neither the polyimide nor the polysiloxane is disclosed, however.
European Patent Application 0 284 803 published October 5, 1988, discloses fully imidized polyimidosiloxanes derived from oxydiphthalic anhydride, a difunctional siloxane monomer and an organic diamine for use in wire and cable coating and for preparing film, fibers and molded and extruded articles. The compositions are not disclosed as having improved atomic oxygen resistance.
U.S. application Ser. No. 606,430, filed May 2, 1984, now abandoned to NASA discloses a flexible laminate comprising a polyimide layer coated with a silicone composition for use as an atomic oxygen degradation resistant support for a solar cell array in outer space.